Media retention screens allow the passage of fluids but prevent passage of particles greater than a particular size. In some industrial processes, it is desirable to have media retention screens which can carry very high loads. These are typically required at the bottom of a down flow reactor. Steel plates, usually stainless steel plates, are used for this purpose. However, provision must be made for processed fluids to flow through the steel plate and this is most typically done by installing a number of screen nozzles uniformly across the plate. These are usually attached to the plate with threaded end fittings known as nipples, and are typically cylindrical and about 50 mm in diameter. The cylindrical surface of the nozzle defines a series of openings which allow liquids to pass but prevent the flow of particles having a diameter greater than the narrowest part of the slot opening. In one known design, the cylindrical surface defining the openings is formed by a stack of stainless steel rings having a triangular cross section known as screen element, sandwiched between a bottom cover and a top cover both of which are welded to the screen element. A stainless steel threaded nipple is welded to the bottom cover and the screen element is fixed to the steel plate by screwing the nipple into a correspondingly threaded aperture in the steel plate.
There are a number of problems with existing screen nozzles, the first of which arises because the plate and the screen nozzle are both made of stainless steel since they need to be corrosion resistant. Because both the nozzles and plate are made of stainless steel, the nozzles tend to bind to the steel plate and once attached to the steel plate are very difficult to remove. This is a problem as the screen nozzle may be filtering corrosive materials which may result in damage to the screen necessitating replacement of the nozzle. Also, screen nozzles are often cleaned by back flushing with acid. Again this can damage the nozzle even though it is made of stainless steel. The welds on the exterior of the steel nozzle are also a particular area of weakness and often corrode after time. Further, the act of welding the plates to the screen, may also partially block the screen and distort the apertures in the screen.
It is an object of the present invention to provide an improved screen nozzle which addresses and attempts to alleviate at least some of the problems of the prior art screen nozzles discussed above.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.